I have a small house and the AC is run via a single long supply run with short runs off of it to the individual vents. The vents on the second half of the run barely have any air coming out, and so that half of the house is noticeably hotter, while the other have is staying really cool because that's where the thermostat is. I tried completely shutting the vents on that side of the house to see if it would push more to the other half and it didn't seem to make a difference. I've been in the attic and looked for baffles on the supply or the short runs and didn't see anything noticeable (square ducting if that makes a difference). The supply has 2 90 angles on it, but two of the vents that are blowing well are after the second 90, so don't think that part is a problem either.

Any ideas on anything else I should check before calling an AC service company out?

Not in the HVAC biz so my Guesses:

• A duct volume key - for which you searched - is not always readily visible as attached photo and one may have to dig a bit for it
• 
• Your blower fan may be either dirty or going out, needs replacement as our lifespans have been 10-18 years
• A gaping hole is hiding in the square duct somewhere near where pressure drops off and insulation is concealing it & its tell tale cool draft ... mice or squirrels in attic? Old world "duct tape" is no longer CODE compliant ever since the old types came loose = leaks
• A DIY light brush cleaning from register into duct ~24-in might be in order
• Renters will sometimes stuff towels up in duct exits in rooms not used so that most used rooms get more AC
• Decipher the tonnage, determine the SF and see if the design is anywhere close to 450-600sf/ton

Sounds like your system is a rigid duct board trunk design. There will not be baffles or dampers in a system like this. Depending on the age of the duct I have seen where the fiberglass becomes separated from the foil backing and will fall and effectively close the duct, but it's not very easy to spot in the attic if you don't know what you're looking for. If the duct board (house) is over 20 years old this very well could be the issue. Alternative issues would be a dirty coil or filter, a blower motor not running at full speed, a coil that's icing up restricting airflow, etc. The way you can check your duct plenum for what I'm talking about it to take a screw driver in the attic and every few feet tap on the duct gently with the handle, if it's a solid sound you're good, if not, you're looking at a new duct system this fall.

That's going to be a design problem. Plenum and ducting, it turns out, has calculations and design parameters just like heat loads. So, I think your system may simply not have enough air to go around. The AC guys need to give it a look and see how to fix it.

Ecobee thermostat and remote sensors

It gets the rooms to the desired temp a hell of a lot cheaper than replacing ducts.

Ignore hoosier-daddy. He has no clue what he's talking about.

Based on all the descriptions you've given, if there is no damper closed (would be at the flex duct takeoff), then my next place to look would be at the flex duct itself. Is it stretched taught or is there some sagging or bunching in it? You'd be surprised at how much a non-taught flex duct can inhibit air flow. If that's not it, then you may have to call an AC guy.

BTW the tstat location is not really the issue if you have noticeably less air coming out of some grills. It's the lack of air flow more than the tstat location. BTW, the tstat should be located in close proximity to yhe return grill and not on the other side of a door, and not getting sunlight through a window. It's probably fine.

Also, if the hot rooms are on the west side, you've probably got more of the late afternoon heat load there and need more air (not less) than the others. I once solved a similar problem by adding a second duct and ceiling grill in that room. If that's not something you feel comfortable doing yourself, then call an AC guy.

So I have something similar to this going on but the 2 hotter rooms are : 1. A room with two exterior walls and 2. A room with 2 exterior walls and a massive vaulted attic space over it. To me that is just telling me that maybe the insulation during construction wasn't very good. Does that seem reasonable?

Quite reasonable to think that lacking insulation is the issue. Experience would tend to lean toward a design issue. If the air flow out of the vent is cold and you can feel it blowing decently hard then it is most likely a design issues. We would recommend having an actual CFM reading taken using a flow hood and compare that to the designed CFM of the duct size. If it's at capacity or close to it then we can be fairly sure that you do not have enough ducting to the area. We see this a lot with exterior rooms and rooms with large windows. If it was an insulation lacking, you could take your hand or an IR temp gun and feel the walls and find a warm spot most likely. Hope this helps.

TruService

Sea Speed said:

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So I have something similar to this going on but the 2 hotter rooms are : 1. A room with two exterior walls and 2. A room with 2 exterior walls and a massive vaulted attic space over it. To me that is just telling me that maybe the insulation during construction wasn't very good. Does that seem reasonable?

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And shutting other ducts in the main line would increase air flow in to that room and should fix the problem, or is that not the case? You are redirecting air flow so more goes to warmer areas.

I know it was discussed here previously but didn't read thoroughly.

That is a trick question. Shutting down other vents can increase the amount of air into the other area to a point. The point being where the duct physically can't handle any more air, the area you just took air from becomes uncomfortable, the increase in internal static exceeds what the blower motor can handle, etc.

I'd be very careful about just adjusting other dampers and at a minimum mark where the handles are before you start so you know where to go back to. Duct balancing is a trial and error system in most cases. I've seen too many times where balancing doesn't fix anything because the design is wrong. We have a dentist office in town where an 8x8 duct trunk is feeding two 8" flex, one 6" flex and one 5" flex. The trunk can't supply half of that total air needed. The system has been this way since it was built and the spaces served by the unit have been hot since day one. The 8x8 might feed one 8" and one 5" flex okay.

So I say all that to warn you to be careful of what you do as it may make things worse.

TruService

Sea Speed said:

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And shutting other ducts in the main line would increase air flow in to that room and should fix the problem, or is that not the case? You are redirecting air flow so more goes to warmer areas.

I know it was discussed here previously but didn't read thoroughly.

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Sea Speed said:

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So I have something similar to this going on but the 2 hotter rooms are : 1. A room with two exterior walls and 2. A room with 2 exterior walls and a massive vaulted attic space over it. To me that is just telling me that maybe the insulation during construction wasn't very good. Does that seem reasonable?

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I had a room over a garage that was always hot in summer and cold in winter. It was just so much more surface area exposure than other rooms in the house with 5 of the 6 sides exposed (including the floo). Besides adding the second duct, I was able to get access to two walls above the garage, so I added insulation there and in the attic. All that solved the problem. So adding extra insulation in your attic over the room will help. It may not solve the problem completely, but it will always help by a marginal amount.